damn, so many things stated, but some of the obvious things were left unsaid
1. Keep the deck under you and your feet (not in front, behind, to the side, centered under you)
2. Keep the deck parallel/flat to the ground
3. This is to say when you jump, you should feel the centered feeling of a double whip flat because the deck is straight and centered it's under you, it will just feel really right, and once you get there...it's all wrists
4. I had the same problem at one point, I think it's just all habit (muscle memory) and maybe for some, some strength, but if you're getting them off tiny ramps, just work your way down, If you can double off a 4 inch curb, inevitably, with practice, you will just naturally learn to double flat without a bunch of effort
5. You have the tips, but when it comes to tricks, there is never a recipe for success. Help is just from advice, never will you hear info that will allow you to go and just land a trick (granted it has any difficulty) Now that you have an idea on what to do, PRACTICE PRACTICE PRACTICE PRACTICE PRACTICE
good luck dude, keep us posted